Method and system of a controllable tail buoy

ABSTRACT

Controllable tail buoy. At least some of the illustrative embodiments are methods including: towing a sensor streamer and tail buoy through water, the sensor streamer defining a proximal end and a distal end with the tail buoy coupled to the distal end, and the towing with the sensor streamer and the tail buoy submerged; and during the towing controlling depth of the distal end of the sensor streamer at least in part by the tail buoy; and steering the distal end of the sensor streamer at least in part by the tail buoy.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of and claims priority under 35 U.S.C.§120 to U.S. patent application Ser. No. 13/198,805 filed Aug. 5, 2011titled “Method and System of a Controllable Tail Buoy,” which isincorporated by reference herein as if reproduced in full below.

BACKGROUND

Marine survey systems are used to acquire data (e.g., seismic,electromagnetic) regarding Earth formations below a body of water suchas a lake or ocean. The marine survey systems comprise a complex arrayof buoys, lines, and paravane systems in order to properly orientstreamers towed behind the survey vessel.

Weather and related sea conditions may adversely affect the ability toperform a marine survey. In adverse weather conditions, the surfacewaves may induce noise in the signals detected by the underwaterstreamers by way of the surface buoys associated with the streamers.Moreover, surface obstacles, such as ships, may interfere with thesurface buoys.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of exemplary embodiments, reference will nowbe made to the accompanying drawings in which:

FIG. 1 shows an overhead view of a marine survey in accordance with atleast some embodiments;

FIG. 2 shows a side elevation view of marine survey in accordance withat least some embodiments;

FIG. 3 shows a partial overhead view of a marine survey, showingsteering of the sensor streamer by the tail buoy, in accordance with atleast some embodiments;

FIG. 4 shows a perspective view of a tail buoy in accordance with atleast some embodiments;

FIG. 5 shows a side elevation, partial cutaway and block diagram, viewof a tail buoy in accordance with at least some embodiments;

FIG. 6 shows a computer system in accordance with at least someembodiments;

FIG. 7 shows a method regarding towing a sensor streamer in accordancewith at least some embodiments; and

FIG. 8 shows a method regarding control of a tail buoy in accordancewith at least some embodiments.

NOTATION AND NOMENCLATURE

Certain terms are used throughout the following description and claimsto refer to particular system components. As one skilled in the art willappreciate, different companies may refer to a component by differentnames. This document does not intend to distinguish between componentsthat differ in name but not function. In the following discussion and inthe claims, the terms “including” and “comprising” are used in anopen-ended fashion, and thus should be interpreted to mean “including,but not limited to . . . . ” Also, the term “couple” or “couples” isintended to mean either an indirect or direct connection. Thus, if afirst device couples to a second device, that connection may be througha direct connection or through an indirect connection via other devicesand connections.

“Cable” shall mean a flexible, axial load carrying member that alsocomprises electrical conductors and/or optical conductors for carryingelectrical power and/or signals between components.

“Rope” shall mean a flexible, axial load carrying member that does notinclude electrical and/or optical conductors. Such a rope may be madefrom fiber, steel, other high strength material, chain, or combinationsof such materials.

“Line” shall mean either a rope or a cable.

“Submerged” shall mean that an object resides fully below the surface ofthe water. If any portion of the object resides above the surface, thenthe object shall not be considered submerged. “Submerges” shall meanthat an object becomes submerged.

“Buoyancy” of an object shall refer to buoyancy of the object takinginto account any weight supported by the object.

“Chord” shall mean an imaginary straight line joining a leading edge anda trailing edge of a surface along the direction of travel when in usein a marine survey.

DETAILED DESCRIPTION

The following discussion is directed to various embodiments of theinvention. Although one or more of these embodiments may be preferred,the embodiments disclosed should not be interpreted, or otherwise used,as limiting the scope of the disclosure or the claims. In addition, oneskilled in the art will understand that the following description hasbroad application, and the discussion of any embodiment is meant only tobe exemplary of that embodiment, and not intended to intimate that thescope of the disclosure or the claims, is limited to that embodiment.

The various embodiments are directed to a tail buoy for streamers towedbehind a survey vessel during a marine survey. More particularly, thevarious embodiments are directed to a tail buoy where the tail buoy maybe selectively submerged, thus reducing the amount of movement of thebuoy caused by surface chop and avoiding surface obstacles. In at leastsome embodiments the tail buoy also steers the distal end of the sensorstreamers (e.g., to help avoid entanglement with other sensorstreamers). Moreover, a tail buoy in accordance with at least someembodiments may be “instrumented” to contain a variety of electricaland/or electromechanical instruments directly or indirectly useful inconducting marine surveys. The specification first turns to anillustrative marine survey system, and then discusses tail buoys inaccordance with various embodiments.

FIG. 1 shows an overhead view of a marine survey system 100 inaccordance with at least some embodiments. In particular, FIG. 1 shows asurvey vessel 102 having onboard equipment 104, such as navigation,energy source control, and data recording equipment. Survey vessel 102is configured to tow one or more sensor streamers 106A-F through thewater. While FIG. 1 illustratively shows six streamers 106, any numberof streamers 106 may be equivalently used.

The streamers 106 are coupled to towing equipment that maintains thestreamers 106 at selected lateral positions with respect to each otherand with respect to the survey vessel 102. The towing equipment maycomprise two paravane tow lines 108A and 108B each coupled to the vessel102 by way of winches 110A and 110B, respectively. The winches enablechanging the deployed length of each paravane tow line 108. The secondend of paravane tow line 108A is coupled to a paravane 112, and thesecond end of paravane tow line 108B is coupled to paravane 114. In eachcase, the tow lines 108A and 108B couple to their respective paravanesthrough respective sets of lines called a “bridle”. The paravanes 112and 114 are each configured to provide a lateral force component to thevarious elements of the survey system when the paravanes are towed inthe water. The combined lateral forces of the paravanes 112 and 114separate the paravanes from each other until the paravanes put one ormore spreader lines 120, coupled between the paravanes 112 and 114, intotension. The paravanes 112 and 114 either couple directly to thespreader line 120, or as illustrated couple to the spreader line by wayof spur lines 122A and 122B.

The streamers 106 are each coupled, at the ends nearest the vessel 102(i.e., the proximal ends) to a respective lead-in cable termination124A-F. The lead-in cable terminations 124 are coupled to or areassociated with the spreader lines 120 so as to control the lateralpositions of the streamers 106 with respect to each other and withrespect to the vessel 102. Electrical and/or optical connections betweenthe appropriate components in the recording system 104 and the sensors(e.g., 109A, 109B) in the streamers 106 may be made using inner lead-incables 126A-F. Much like the tow lines 108 associated with respectivewinches 110, each of the lead-in cables 126 may be deployed by arespective winch or similar spooling device such that the deployedlength of each lead-in cable 126 can be changed.

Although not shown in FIG. 1, each streamer 106A-F may be associatedwith one or more buoys. FIG. 2 shows a side elevation view of a streamer106 in an operational configuration. In particular, FIG. 2 shows astreamer 106 being towed in a direction indicated by arrow 200 by towvessel 102. In some embodiments, the forward portion of the streamer maybe associated with a lead buoy 202, where lead buoy 202 may helpmaintain the depth of the streamer 106 and/or associated portion of thespreader line; however, in other cases the lead buoy 202 may be omitted,or other buoys (e.g., buoys associated with the spreader line 120 (notshown in FIG. 2) may perform similar functions. Although the streamerharness or bridle arrangement of FIG. 1 has been described forillustrative purposes, other arrangements may be used without deviatingfrom the scope of the invention as described and claimed below.

FIG. 2 also illustrates a tail buoy 204. Tail buoy 204 may couple to thesensor streamer 106 by any suitable mechanism, such as line 206,sometimes referred as a “dead section”. The line 206 may have anysuitable length, and in some cases between 50 and 250 meters. Tail buoy204 may serve several purposes. For example, when at the surface (shownin dashed lines) tail buoy 204 may serve as a visual indication of thelocation of the end of the streamer 106. In some cases, the tail buoy204 may at least partially support the sensor streamer 106. Inparticular, the streamer 106 may be configured to be neutrally buoyant,or perhaps very slightly negatively buoyant depending on the salinityand temperature of the surrounding water. In a particular embodiment,each tail buoy 204 may provide support on the order of 30 kilograms ormore to its attached sensor streamers. Other supported weights arepossible. Thus, tail buoy 206 may help maintain the depth of thestreamer 106.

However, being mechanically coupled to the streamer 106, when the tailbuoy 204 is at the surface the tail buoy 204 may impart unwanted motionto the streamer 106, particularly in choppy seas. Such unwanted motionmay result in noise in the signals detected by the sensors of thestreamers. In order to reduce the amount of motion in the streamer 106induced by the tail buoy 204, in accordance with various embodiments thetail buoy 204 may be selectively submerged. Operating the tail buoysubmerged may expose the buoy to less surface chop, and may thus induceless noise in the readings taken by the sensor streamer 106. Moreover,the tail buoy 204 may be submerged to avoid obstacles, such as otherships, or sensor streamers (on the same or different marine surveysystems). Even in the submerged state, the tail buoy 204 may at leastpartially support the sensor streamer 106.

The commands to cause the tail buoy 204 to submerge or surface may havemany forms. In some embodiments, the tail buoy 204 is communicativelycoupled to the tow vessel 102 by way of the lead-in cable 126, sensorstreamer 106, and line 206 in the form of a cable. In yet still otherembodiments, the tail buoy 204 may be communicatively coupled to asurface vessel (e.g., the tow vessel 102) by way of an acousticcommunication system that utilizes the water as the communicationmedium. For example, in FIG. 2 the tow vessel 102 is illustrated toproduce acoustic signals in the form of pressure waves 210 within thewater. The pressure waves 210 propagate to and are received by the tailbuoy 204. The pressure waves 210 may encode commands, like commands forthe tail buoy 204 to submerge, or to submerge to a particular depth.Other commands are possible, such as commands regarding steering thedistal end of the sensor streamer 106 (discussed more with respect FIG.3). While in some cases the tail buoy 204 may be configured only toreceive and implement commands, in other cases the tail buoy 204 may becapable of acoustically communicating with the other vessels (e.g., towvessel 102), as illustrated by pressure waves 220 emanating from thetail buoy 204. In yet still other cases, the tail buoy 204 may becommunicatively coupled by electromagnetic waves through air or water,such as satellite-based communications or locally broadcast signals.

In addition to the ability to selectively submerge, a tail buoy 204 inaccordance with various embodiments may also change its heading throughthe water in order to fully or partially steer the distal end of thesensor streamer 106. FIG. 3 shows an overhead view of single sensorstreamer 106 and tail buoy 204 in order to more fully describe theeffects of the steering. In particular, FIG. 3 shows sensor streamer 106with a deviation 300 from a desired course 301 (dashed lines). Thedeviation may be caused by a variety of factors, such as cross-currentsin the water within which the marine survey is taking place. FIG. 3 alsoshows tail buoy 204 with a heading 302 (shown in dash-dot-dash line)that tends to pull or steer the sensor streamer 106. More particularly,the tail buoy 204 exerts a force in the direction indicated by arrow 304(the force on the streamer exerted by way of the dead section 206). Theforce 304 thus tends to move the sensor streamer 304 in the direction ofthe force, and in this illustrative case toward the desired course 301.

The various angles and relationships in FIG. 3 may be exaggerated forpurposes of explanation. The sensor streamer 106 may be from less than2000 to in excess of 12000 meters in length in some cases, and thusrelatively small angle deviations may result in the distal end of asensor streamer being tens of meters off the desired track. Moreover, asmentioned above the dead section 206 may be in the range of 50 to 250meters and length, and in operation (i.e., as the sensor streamer 106 istowed through the water) the actual amount of deviation of the tail buoy204 during the steering maneuver may be relatively small, in some casesfrom three to five degrees measured from a central axis of the sensorstreamer. Nevertheless, the amount of horizontal or lateral force thetail buoy 204 can supply, possibly in combination with steeringmechanisms directly coupled to the sensor streamer 106, may besufficient to cause and/or correct a change in track of the sensorstreamer 106.

FIG. 4 shows a perspective view of a tail buoy 204 in accordance with atleast some embodiments. In particular, the tail buoy 204 comprises anelongated outer body 400 that defines a forward portion 402 and an aftportion 404. The elongated outer body may define a circularcross-section as illustrated, but other cross-sectional shapes may beused. The forward portion 402 is shown rounded to present an efficienthydrodynamic shape (i.e., to reduce drag), but other shapes for theforward portion 402 may be used. A tow point 406 is coupled to theelongated outer body 400, and as illustrated the tow point 400 may becoupled to the forward portion 402. The tow point 406 may be thelocation at which the line 206 is coupled. Other tow points disposed atother locations on tail buoy 204 may also be used. Illustrative tailbuoy 204 may further comprise a vertical stabilizer 408 and rudder 410within the vertical stabilizer 408. Vertical stabilizer is labeled“vertical” because of its illustrative orientation. When the tail buoy204 is towed through water, the vertical stabilizer 408 acts tostabilize the yaw (i.e., rotation about a vertical axis 412, therotation illustrated by double-headed arrow 414). Rudder 410 may bedeflected, as illustrated by double-headed arrow 416. Deflection ofrudder 410 results in forces tending to change the yaw of the tail buoy204, and thus change the heading of the tail buoy 204 (e.g., to exertforces to steer an attached sensor streamer). Although illustrative FIG.4 shows a separate vertical stabilizer 408 and rudder 410, in some casesthe vertical stabilizer and rudder are the same structure, withdeflection of the entire structure developing the forces tending tochange the yaw of the tail buoy 400. For example, the entire verticalstabilizer may rotate about a vertical axis.

The illustrative tail buoy 204 also has a horizontal stabilizer 418,illustratively shown coupled on a distal end of the vertical stabilizer408. In other embodiments, the horizontal stabilizer 418 may couple atthe proximal end of the vertical stabilizer (i.e., closer to theelongated outer body 400), or may couple directly to the elongated outerbody 400. Horizontal stabilizer 418 is labeled “horizontal” because ofits illustrative orientation. When the tail buoy 204 is towed throughwater, the horizontal stabilizer 418 acts to stabilize the pitch (i.e.,rotation about the horizontal axis 420, the rotation illustrated bydouble-headed arrow 422). In some cases, the horizontal stabilizer maybe a solid structure (implementing no changes in pitch); however, in yetstill further embodiments the horizontal stabilizer 418 comprises acontrol surface 424 which may be deflected, as illustrated bydouble-headed arrow 426. Deflection of control surface 424 results inforces tending to change the pitch of the tail buoy 204, which mayresult in a change in depth of the buoy 204. Although illustrative FIG.4 shows a separate horizontal stabilizer 418 and control surface 424, insome cases the horizontal stabilizer and control surface are the same,with deflection of the entire structure developing the forces tending tochange the pitch. For example, the entire horizontal stabilizer mayrotate about a horizontal axis.

The horizontal stabilizer 418 may be omitted in some embodiments.Changes in pitch may also be implemented by changing buoyancy of thetail buoy 204. For example, making the forward portion 402 morepositively buoyant than the aft portion 404 may result in pitch-uporientation. Likewise, making the aft portion 404 more positivelybuoyant than the forward portion 402 may result in pitch-downorientation. In other cases, the control system for the tail buoy 204(the control system discussed below) may simultaneously control buoyancyand deflection of the control surface 426 to implement pitch changes.

Still referring to FIG. 4, the illustrative tail buoy 204 furthercomprises a set of wings 430 coupled to and extending away from theelongated outer body 400. In particular, the set of wings 430 comprisesa port wing 432 and a starboard wing 434. When the tail buoy 204 istowed through water, the set of wings 430 acts to stabilize the roll(i.e., rotation about the long axis 435, the rotation illustrated bydouble-headed arrow 436). In some cases, the wing 432 and 434 may be asolid structure (implementing no changes in roll); however, in yet stillfurther embodiments the each wing of the set of wings 430 comprises acontrol surface which may be deflected. For example wing 432 mayimplement control surface 438 whose deflection is illustrated bydouble-headed arrow 440, and wing 434 may implement control surface 442whose deflection is illustrated by double-headed arrow 444. Deflectionof the control surfaces results in a change in lift of the respectivewing, and thus (when deflected oppositely) may result in forces tendingto change the roll of the tail buoy 204. Although illustrative FIG. 4shows separate wings 432, 434 and control surfaces 440, 442respectively, in some cases the wings and control surface the same, withdeflection of the entire wing developing the forces tending to changethe roll. For example, the wings may rotate (oppositely) abouthorizontal axis 420, which may result in a change in roll.

In one embodiment, each wing defines a cross-section in the form of asymmetric airfoil. In the case of a symmetric airfoil, water flow overand under the wing experiences the same travel distance, and thus whileproviding a stabilizing force, no net lift is created. In yet stillother embodiments, each wing defines a cross-section in the form of anon-symmetric airfoil. In the case of a non-symmetric airfoil, waterflow over and under the wing experience different travel distance, andthe faster path results in lower pressure (Bernoulli's principle) andthus lift is created (assuming the longer distance is over the uppersurface) where the net lift force is roughly parallel to the verticalcentral axis 412.

Implementing the set of wings 430 as non-symmetric airfoils, therebycreating lift as the tail buoy 204 is towed through the water, enablesseveral operational modes. In some cases, the lift provided may helpsupport the attached sensor streamer. That is, the lift may provide thelifting force without, or with reduced, reliance on the buoyancy of thetail buoy 204. Moreover, when roll changes are implemented, thehorizontal component of the lift force developed may assist in turningthe tail buoy 204 (possibly in combination with the rudder 410deflection), thus increasing the amount of force the tail buoy 204 canimpart to the sensor streamer by way of the line 206.

The force developed by the set of wings 430 having a non-symmetricairfoil shape need not be a lifting force in all cases. In otherembodiments, the longer path length may be under the wing, such that theforce developed by water flow relative to the set of wings 430 mayresult in a force tending to submerge the tail buoy 204. That is, insome embodiments the tail buoy 204 may be configured to be positivelybuoyant, thus floating at the surface when not being towed. However,when the tail buoy is towed, the force developed by water flow relativeto the set of wings 430 may tend to submerge the tail buoy 204.

FIG. 5 shows a side elevation view, with partial cutaway, to showinternal components (in block diagram form). In particular, FIG. 5 showsthat the elongated outer body 400 may define an interior volume 500.Within the interior volume 500 may reside a buoy control system 502coupled to: an acoustic communication system 504 (“acoustic comsystem”); a front buoyancy control system 506; a pitch deflection system508; a wing deflection system 510; an aft buoyancy control system 512; arudder deflection system 514; an instrumentation system 516; and asatellite communication system (“Sat Sys”) 518. Each of the coupledsystems may reside at least partially within the interior volume 500.The acoustic communication system 504 may be used to receive commands(e.g., commands to submerge, surface, or to steer an attached sensorstreamer). In some cases, the buoy control system 502 may alsocommunicate with other vessels (e.g., the tow vessel) by way of theacoustic communication system.

The buoy control system 502 further illustratively couples to theforward buoyancy control system 506 and aft buoyancy control system 512.Thus, the buoy control system 502 may command the buoyancy controlsystem 506 and 512 to implement desired changes in buoyancy. Eachbuoyancy control system 506 and 512 sets the buoyancy of the respectiveportion of the tail buoy 204. In some cases, the buoyancy controlsystems 504 and 512 can be operated in unison, which may result inchanges of depth without resulting in substantial pitch changes. Inother cases, the buoyancy control systems may be operated at differentrates, or resulting in opposite changes in buoyancy, to implement orassist in implementing pitch changes of the tail buoy 204. The buoyancycontrol implemented by the buoyancy control systems 504 and 512 may takeany suitable form. In some cases, buoyancy control is implemented by wayof a piston and cylinder arrangement, wherein when the piston moves inone direction a gas is compressed and water enabled to enter theinterior volume, and when the piston moves in the opposite directionwater is displaced from the interior volume 500. Other suitablemechanisms, such as water pumps and ballast systems, may be used. Thebuoyancy control implemented by the forward buoyancy control system 506need not be the same as that implemented by the aft buoyancy controlsystem 512.

Still referring to FIG. 5, the buoy control system 502 is operativelycoupled to the pitch deflection system 508. The pitch deflection system508 enables changes in control surface 424 of the horizontal stabilizer418. The pitch deflection system 508 may take any suitable form, such aswires or ropes mechanically coupled to a deflection member, such asmotor or piston.

The buoy control system 502 is operatively coupled to the wingdeflection system 510. In some cases, a single wing deflection system510 may be present, which simultaneously and oppositely operates bothcontrol surfaces 438 and 442. In other cases, the control surfaces maybe independently operated by way of distinct wing deflection systems.The wing deflection system 510 may take any suitable form, such as wiresor ropes mechanically coupled to a deflection member, such as motor orpiston.

The buoy control system 502 is operatively coupled to the rudderdeflection system 514. The rudder deflection system 514 enables changesin rudder 410 position. The rudder deflection system 514 may take anysuitable form, such as wires or ropes mechanically coupled to adeflection member, such as motor or piston.

Still referring to FIG. 5, the buoy control system 502 may further beoperatively coupled to instrumentation system 516. The composition ofthe instrumentation system 516 may take many forms. In some cases, theinstrumentation system 516 may comprise an inertial navigation system.That is, during periods of time when the tail buoy 204 is submerged andbeing towed through the water, the instrumentation system 516 in theform of an inertial navigation system may estimate the track of the tailbuoy (e.g., using devices such as accelerometers, inclinometers, and/ordirectional gyros). The estimated track created by the instrumentationsystem 516 in the form of an inertial navigation system may be usefulnot only with respect to knowing the location at various points in timeof the attached sensor streamer, but also may be useful with respect tosteering the attached sensor streamer. In some cases, theinstrumentation system 516 in the form of an inertial navigation systemcreates the estimated track, and then sends the estimated track to asurface computer by any suitable means, such as the acousticcommunication system 504, or the satellite communication system 518 whenthe tail buoy is at the surface of the water. That is, the estimatedtrack may be encoded in electromagnetic radiation and sent to a surfacecomputer during periods of time when tail buoy 204 is on the surface.

The instrumentation system 516 is not limited to just an inertialnavigation system. Other types of instrumentation may be used in placeof, or in addition to, an inertial navigation system. For example, theinstrumentation system 516 may comprise sensors to read the salinity andtemperature of water surrounding the tail buoy 204. In other cases, theinstrumentation system 516 may comprises devices complementary to themarine survey, such as bottom profilers for reading the topography ofthe ocean bottom below tail buoy, and magnetometers for reading changesin magnetic field.

The body of the tail buoy 204 may be constructed of any suitablematerial. In some cases the tail buoy is constructed of a plasticmaterial, perhaps over a rigid internal metallic structure (e.g.,aluminum). In other cases, the tail buoy 204 may be constructed of selfsupporting material, such as a carbon composite material or fiberglass,such that no internal structure is needed. In yet still further cases,the tail buoy 204 may be constructed of metallic material (e.g., steel,aluminum, or alloys). Further still, the body of the tail buoy 204 maymade from combinations of different material for different parts of thebuoy (e.g., the elongated outer body constructed from one material, andthe wings and stabilizers constructed from a different material).

In accordance with at least some embodiments, the buoy control system502 is a computer system executing a program. FIG. 6 shows an electricalblock diagram of buoy control system 502 in accordance with at leastsome embodiments. In particular, the buoy control system 502 comprises aprocessor 600 coupled to a program storage memory 602 and input/outputdevices 604 by way of one or more communication buses 606. The processor600 may take any suitable form, and depending on the amount ofprocessing power used by the buoy control system, the processor 600 maybe multiple processors, or processors with multiple cores. In othercases, particularly cases where the buoy control system 502 operates ona limited energy supply like a battery, the processor 600 may be aprocessor with limited processing capability, and implementing variouspower saving features (e.g., sleep modes, reduced power operationalstates).

The program storage memory 602 may take any suitable form, such asrandom access memory (RAM), read only memory (ROM), or a long termstorage device (e.g., flash memory, hard disk drive). The programstorage memory 602, which is an example of a non-transitorycomputer-readable medium, may thus store programs executed by theprocessor 600 to implement the various embodiments discussed above. TheI/O devices 604 likewise may take any suitable form, such as parallelcommunication ports, serial communication ports, analog input ports,analog output ports, digital input ports, and digital output ports.

In some cases, the buoy control system 502 may be implemented asindividual processor 600, program storage memory 602, and I/O devices604; however, in yet still other embodiments the processor, memory andI/O functionality may be implemented by way of an integrated unit, suchas a microcontroller available from any suitable source.

FIG. 7 shows a method in accordance with at least some embodiments. Inparticular, the method starts (block 700) and comprises towing a sensorstreamer and tail buoy through water, the sensor streamer defining aproximal end and a distal end with the tail buoy coupled to the distalend, and the towing with the sensor streamer and the tail buoy submerged(block 702). During the towing, the method further comprises:controlling depth of the distal end of the sensor streamer at least inpart by the tail buoy (block 704); and steering the distal end of thesensor streamer at least in part by the tail buoy (block 706).Thereafter, the method ends (block 708), in some cases to be repeated.

FIG. 8 shows a method that may be implemented by way of software. Inparticular, the method starts (block 800) and comprises: receivingcommands from an acoustic communication system (block 802); andresponsive to the commands changing buoyancy of the tail buoy bycommunication with the buoyancy control system (block 804); and changingheading of the tail buoy by communication with the rudder deflectionsystem (block 806). Thereafter the method ends (block 808), in somecases to be repeated.

References to “one embodiment”, “an embodiment”, “a particularembodiment”, and “some embodiments” indicate that a particular elementor characteristic is included in at least one embodiment of theinvention. Although the phrases “in one embodiment”, “an embodiment”, “aparticular embodiment”, and “some embodiments” may appear in variousplaces, these do not necessarily refer to the same embodiment.

The above discussion is meant to be illustrative of the principles andvarious embodiments of the present invention. Numerous variations andmodifications will become apparent to those skilled in the art once theabove disclosure is fully appreciated. For example, while no specificpower source is discussed, the power source for the tail buoy could takemany suitable forms, such as batteries, and/or electrical generatorsthat produce power based on relative water flow past the tail buoy. Itis intended that the following claims be interpreted to embrace all suchvariations and modifications.

What is claimed is:
 1. A tail buoy comprising: an elongated outer bodydefining a forward portion, an aft portion, an interior volume, and acentral axis; a tow point coupled to the elongated outer body on theforward portion; a rudder coupled to and extending away from theelongated outer body; a rudder deflection system disposed at leastpartially within the elongated outer housing, the rudder deflectionsystem operatively coupled to the rudder and configured to selectivelydeflect the rudder; a set of wings coupled to and extending away fromthe elongated outer body, the set of wings comprising a first wing and asecond wing; a first control surface associated with the first wing, thefirst control surface configured to change lift of the first wing; asecond control surface associated with the second wing, the secondcontrol surface configured to change lift of the second wing; a firststabilizer disposed proximate the aft portion of the elongated outerbody, at least a portion of the first stabilizer configured to deflectto cause changes in pitch of the elongated outer body; a buoyancycontrol system disposed at least partially within the interior volume,the buoyancy control system configured to selectively set buoyancy ofthe tail buoy at a value within a range of values between positivelybuoyant and negatively buoyant; an acoustic communication systemdisposed at least partially within the interior volume and configured toreceive messages from a remote device by way of pressure waves in water;a buoy control system disposed at least partially within the elongatedouter body, the buoy control system operatively coupled to the rudderdeflection system, the buoyancy control system, and the acousticcommunication system, the control system comprising: a processor; and amemory coupled to the processor; wherein the memory stores a programthat, when executed by the processor causes the processor to: receivecommands from the acoustic communication system; and responsive to thecommands change buoyancy of the tail buoy by communication with thebuoyancy control system; and change heading of the tail buoy bycommunication with the rudder deflection system.
 2. The tail buoy ofclaim 1 wherein the processor is configured to submerge the tail buoy bychanging buoyancy of the tail buoy.
 3. The tail buoy of claim 1 whereinthe processor is configured to change depth of the tail buoy by changingbuoyancy of the tail buoy.
 4. The tail buoy of claim 1 furthercomprising: a first deflection control system operatively coupled to thefirst control surface, the first deflection control system configured toselectively deflect the first control surface, and the first deflectioncontrol system communicatively coupled to the buoy control system; asecond deflection control system operatively coupled to the secondcontrol surface, the second deflection control system configured toselectively deflect the second control surface, and the seconddeflection control system communicatively coupled to the buoy controlsystem; wherein the program is configured to cause the processor tochange heading of the tail buoy by communication with the first andsecond deflection control system.
 5. The tail buoy of claim 1 furthercomprising an inertial navigation system disposed at least partiallywithin the elongated outer housing and communicatively coupled to thebuoy control system, the inertial navigation system configured toestimate a track of the tail buoy when submerged.
 6. The tail buoy ofclaim 5 further comprising a satellite communication system disposed atleast partially within the elongated outer housing and communicativelycoupled to the buoy control system, the satellite communication systemconfigured to transmit the estimated track when the tail buoy is at thesurface.
 7. The tail buoy of claim 1 wherein the cross-sectional shapeof the first wing and the second wing are each a non-symmetric airfoil.8. The tail buoy of claim 7 wherein the cross-sectional shape of thefirst and second wings are configured to provide a force tending tosubmerge the tail buoy as the tail buoy is towed through the water. 9.The tail buoy of claim 8 wherein the tail buoy is positively buoyant.10. The tail buoy of claim 7 wherein cross-sectional shape of the firstand second wings are configured to provide a force tending to cause thetail buoy to surface as the tail buoy is towed through the water. 11.The tail buoy of claim 1 further comprising a sensor at least partiallydisposed within the elongated outer body, the sensor configured to readsalinity of water proximate the tail buoy.
 12. The tail buoy of claim 1further comprising a sensor at least partially disposed within theelongated outer body, the sensor configured to read temperature of waterproximate the tail buoy.
 13. The tail buoy of claim 1 further comprisinga bottom profiler at least partially disposed within the elongated outerbody, the bottom profiler configured to read topography of an oceanbottom beneath the tail buoy.